But except for a polling uptick after his strong Iowa finish, Mr. Buttigieg’s support in an average of national polls plateaued around 10 percent. That imperiled him as the race moved to the 14 Super Tuesday states, including California and Texas, where most delegates to the National Convention go only to candidates who win 15 percent in congressional districts and statewide.
As Mr. Sanders, in his second presidential run, built a devoted following of progressives with a call for political revolution, Mr. Buttigieg tried to offer an alternative: an upbeat message of unity and more ideological flexibility, aimed at attracting moderate Democrats, independents and crossover Republicans. But the pitch, which some found contained more platitudes than passion, was no match at a time of rising anger on the left that the political establishment has failed to address health care, income inequality and climate change.
In his quest to earn black support, Mr. Buttigieg spent more time visiting South Carolina than any other candidate, spent more on TV ads in the state than any candidates besides the billionaire Tom Steyer, and rolled out a sweeping proposal, called the Douglass Plan, to redress the legacy of racism. None of it made much of a dent with African-American voters who had developed a deep trust in Mr. Biden over decades.
Another factor may have been the sometimes troubled history of Mr. Buttigieg’s relationship with black residents of South Bend, including his demotion of a black police chief and the shooting last summer of a black resident by a white officer. Mr. Buttigieg tried to counter poor impressions by campaigning with African-American leaders from his hometown who vouched for him.
All along, he believed that winning in Iowa would beget winning in later states with more racially diverse voters.
Despite an early exit from the race, Mr. Buttigieg’s candidacy will be remembered for its remarkably high trajectory: the mayor of Indiana’s fourth-largest city outran, out-raised and outpolled senators and governors who dropped by the wayside.
Mr. Buttigieg’s decision, just before Super Tuesday, echoed one he made three years ago during his first foray into national politics. In late February 2017, Mr. Buttigieg dropped out of the contest to become chairman of the Democratic National Committee on the morning of the vote after it became clear he had commitments from fewer than 10 D.N.C. members. After his withdrawal, Mr. Buttigieg received a single vote, from Mayor Nan Whaley of Dayton, Ohio.